This application is related to Japanese Patent Application No. 2001-199219 filed in Jun. 29, 2001, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave device, and more particularly, it relates to a surface acoustic device comprising a piezoelectric substrate having thereon an electrode for exciting a surface acoustic wave.
2. Description of the Related Art
Filters and resonators utilizing a surface acoustic wave (SAW) have such characteristics that they are small-sized and inexpensive. Therefore, today, a surface acoustic wave device is widely used in a band-pass filter and a duplexer of a communication equipment, such as a portable telephone.
In recent years, a filter using the surface acoustic wave device is demanded to have high capability along with the advance in performance of the portable telephone and the like. However, there is a problem in that the pass band shifts due to temperature variation, and thus improvement of the temperature stability is demanded.
Lithium tantalate (hereinafter abbreviated as LT), which is frequently used as a material for a substrate of the surface acoustic wave device at present, is a material advantageous for realizing broad band filter characteristics and is a piezoelectric material having a large electromechanical coupling factor. However, it disadvantageously has poor temperature stability in comparison to a quartz substrate. As the general tendency of piezoelectric materials, materials having a large electromechanical coupling factor are inferior in temperature stability, whereas materials excellent in temperature stability, such as quartz, have a disadvantage of a small electromechanical coupling factor.
Various proposals have been made in order to realize a material having a large electromechanical coupling factor and excellent temperature stability.
For example, such a substrate has been proposed that is formed with a lithium niobate (hereinafter abbreviated as LN) or LT substrate having on the whole surface thereof a quartz film having an opposite temperature coefficient (IEEE Trans. on Sonics and Ultrasonics, vol. SU-31, pp. 51 to 57 (1984)).
It has been also proposed that a polarization inversion layer having a thickness equal to or smaller than a SAW wavelength is formed on a surface of an LT substrate, and the temperature stability is improved by utilizing the electric field shunt effect thereof (Japanese Patent No. 2,516,817).
Furthermore, such a surface acoustic wave device has been proposed that is to be improved in temperature stability by coupling a thin piezoelectric substrate and a thick low expansion material substrate through direct coupling to suppress expansion and contraction of the piezoelectric substrate due to temperature variation (Japanese Unexamined Patent Publication No. Hei 11(1999)-55070, Proc. of IEEE Ultrasonics Symposium, pp. 335 to 338 (1998), etc.), and also a device has been proposed that realizes the similar structure by using an adhesive (The 20th Ultrasonic Symposium, November 1999).
Among the proposals having been made for realizing a surface acoustic wave device having a large electromechanical coupling factor and being excellent in temperature stability, the LN or LT substrate having a quartz film formed on the whole surface thereof is difficult to control the film quality of the quartz film to a constant state. Furthermore, because a quartz film is also formed on interdigital transducers (hereinafter abbreviated as IDT) for improving the temperature characteristics, the propagation loss of the surface acoustic wave becomes large, and the device has not yet been put into practical use.
In the method of forming a polarization inversion layer on a surface of an LT substrate, annealing at a high temperature immediately below the Curie point (xe2x89xa6600xc2x0 C.) is necessary, and thus there is a difficulty in controllability of the depth of the polarization inversion layer.
In the method where a low expansion material substrate is coupled with a piezoelectric substrate, the coupling surfaces are necessarily mirror surfaces in order to suppress expansion and contraction due to temperature variation to obtain higher effects. As a result, however, it involves such a problem that reflection of the bulk wave occurs at the coupling interface to affect the surface acoustic wave, whereby the filter characteristics are deteriorated, and thus it has not yet been put into practical use.
In order to solve the problem, Japanese Unexamined Patent Publication No. 2001-53579 proposes a device produced by roughening the back surface of an LT substrate, and then it is coupled with a low expansion material by using an adhesive. However, the effect of improving the temperature stability of the device is lowered along with decrease of the adhesive force at the interface.
The invention provides a surface acoustic wave device comprising a piezoelectric substrate having formed on a surface thereof an interdigital transducer for exciting a surface acoustic wave, a cavity formed on opposite surface of the piezoelectric substrate in a region corresponding to a region where the interdigital transducer is formed, and a low expansion material buried in the cavity.
According to the invention, the temperature stability can be improved without deterioration of the electromechanical coupling factor.
The term xe2x80x9clow expansionxe2x80x9d signifies the thermal expansion coefficient lower than that of the substrate.